Osteoclasts are essential for bone modeling and remodeling and mediate bone loss in common bone diseases, including rheumatoid arthritis and postmenopausal osteoporosis, in which levels of proinflammatory cytokines, such as TNF, are increased and drive osteoclast formation both directly and indirectly through RANKL and three sets of transcription factors: NF-?B; c-Fos; and NFATc1. Expression of these is essential for osteoclast formation. RANKL/RANK signals through a canonical NF-?B1/p65 and an alternative NF-?B2/RelB pathway. To-date, most studies of NF-?B in osteoclasts have focused on the canonical pathway, and much less is known about the functional roles of NF-?B2 and RelB in regulating osteoclasts and other bone cells. Recently, we found that TNF induces NF-?B2 expression and that deletion of NF-?B2 increases TNF-induced osteoclast formation. TNF transgenic/NF-?B2-/- mice develop earlier and more severe joint erosion and inflammation and systemic bone loss than TNF-Tg mice. Furthermore, we found that RelB null mice have mild osteopetrosis. These findings highlight the importance of the alternative pathway in bone and suggest a new role for TNF to limit RANKL-induced bone loss in addition to its known role to stimulate osteoclast formation. Based on our preliminary data, we hypothesize that TNF can induce NF-?B2 expression to limit osteoclast formation stimulated by RANKL and that manipulation of NF-?B2/RelB expression will directly affect osteoclast functions. This hypothesis will be tested in the following 3 Specific Aims. In Aim 1, we will determine if NF-?B2 limits OC formation by controlling signaling in both the canonical and alternative NF-?B pathways. In Aim 2, we will determine the roles of NF-?B2 and RelB in osteoclast formation and activity. In Aim 3, we will determine the effects of over-expression of NF-?B2p100 on TNF-induced bone loss. Our proposed studies should define the roles of NF-?B2 and RelB in osteoclast formation and activity and provide proof of concept that they are strong candidates for novel therapeutic intervention to limit cytokine-stimulated bone loss.